In a GSM cellular system, a Subscriber Identity Module (SIM) contains the identity of the subscriber. One of the primary functions of the SIM in conjunction with the cellular network system is to authenticate the validity of the wireless terminal (for example, a cell phone) and the wireless terminal's subscription to the network. The SIM is typically a microchip that is located on a plastic card which is approximately 1 cm square. The SIM card is then placed in a slot of the wireless terminal to establish the unique identity of the subscriber to the network.
In the SIM of the wireless terminal, an authentication key (Ki)-International Mobile Subscriber Identity (IMSI) pair is stored. This same Ki for a given IMSI is also stored in the Home Location Register (HLR)/Authentication-Center (AuC) of the network. The wireless terminal and the HLR/AuC independently run an authentication algorithm each using a Ki, and if the Ki's are the same for the two, then the authentication will pass.
In existing cellular systems, a wireless terminal has a Ki associated with only one IMSI. Since one of the fields in the IMSI is country code, a wireless terminal is registered as an international roaming device on the network when it is powered on in a network of a country that is different from the one corresponding to the country code in its IMSI. Wireless terminal vendors and their users could greatly benefit if a wireless terminal can be sold in any country and if it can automatically provision itself with an IMSI specific to that country. The wireless terminal vendors could then sell the same generic device in multiple countries and users could then use this terminal without having to pay international roaming charges.